As wells are drilled to greater lengths and depths, it becomes necessary to provide a liner (“casing”) to avoid undesirable fluid inflows or outflows and to prevent borehole collapse. The annular space between the borehole wall and the liner is usually filled with cement (a process referred to as “cementing” the well) to reinforce structural integrity and to prevent fluid flows along the outside of the liner. If such fluid flows are not prevented, there is a loss of zonal isolation. Fluids from high-pressured formations can enter the borehole and travel along the outside of the casing to invade lower-pressured formations, or possibly exit the borehole in a mixture that dilutes the desired production fluid. Results may include contamination of aquifers, damage to the hydrocarbon reservoir, and loss of well profitability.
When cementing a well, the cement is generally injected down the interior of the casing to the bottom of the borehole and forced back upward around the casing. Tools referred to as cementing plugs are sometimes used to separate the cement from spacer fluids injected into the well. Spacer fluids are fluids used to separate and thus reduce contact and mixing between wellbore fluids (e.g., drilling fluid and cement). A lower plug is first inserted into the casing ahead of the cement to separate the cement from spacer fluid already injected into the well. Cement is then pumped into the casing behind the lower plug, which drives the lower plug down into the well. This forces fluid already in the borehole (e.g., spacer and drilling fluid) back up into the annular region between the casing and the formation and to the surface where it is safely collected.
Once the desired amount of cement has been injected into the casing, an upper plug is inserted into the casing, and spacer fluid is injected above the upper plug. The upper plug separates the spacer fluid from the cement, and the two plugs and the cement in between move downward as fluid is injected above the upper plug. As it moves downward, the lower plug wipes fluid and other materials from the inner surface of the casing in front of the cement, thus helping to reduce contamination of the cement. When the lower plug is stopped by a float collar near the bottom of the casing, pressure is increased until a diaphragm in the lower plug ruptures, allowing the cement to flow past the lower plug and float collar, into the bottom of the borehole and back up the annular region outside the casing. Although this puts the cement in contact with the spacer fluid on the other side of the lower plug, the formulation of the spacer fluid reduces the degree to which it mixes with and adversely affects the cement. The upper plug continues to be forced downward by the spacer fluid above it until the cement is forced out from between the two plugs and the upper plug lands on the lower plug. The cement is then left to cure before any further drilling or production activities continue.
As cementing proceeds, it is useful to know the position of each plug as it progresses down the casing in order to track the position of the cement itself. Many existing techniques rely on pressure variations in the fluid to identify the position of the plugs at a few key points during cementing, such as the pressure increase that occurs when the first plug arrives at the float collar. But such pressure variations are generally small enough to be easily missed on the surface (just a few hundred pounds per square inch). Further, other events such as a stuck lower plug may also cause similar pressure increases that may incorrectly be interpreted as a plug reaching a key position. Existing techniques also do not track the distance between the upper and lower plugs (and thus the volume of cement between the plugs) as the plugs and cement travel through the casing. Changes in the cement volume can be indicative of a problem such as significant contamination of the cement. Such contamination can compromise the integrity of the cement, as well as the overall long-term safety of the well and those working around it.
It should be understood that the drawings and corresponding detailed description do not limit the disclosure, but on the contrary, they provide the foundation for understanding all modifications, equivalents, and alternatives falling within the scope of the claims.